It is occasionally necessary to check a user permission in order to execute processing by, for example, an information system composed of client equipment and a server, or an automotive system composed of equipment mounted on an automobile and a server for communicating with such equipment. In these cases, the information system or the automotive system transmits a message to a user to ask for a permission and gets confirmation when the user returns a message of permission. Under this circumstance, the information system or the automotive system can keep a log of these exchanged messages in order to retrospectively explain that the permission has been obtained. However, if validity of the log is questioned, the information system or the automotive system needs to prove the validity of the log. In order to prove the validity, it is only necessary to show that the message of the user permission which is kept in the log was created certainly by the relevant user and has not been falsified. A property in which a creator of such message and its content cannot be denied retrospectively is called “non-repudiation.” An electronic signature technique of public key cryptography technology disclosed in U.S. Pat. No. 5,231,668 is known as a technique that enables the non-repudiation. The technique in U.S. Pat. No. 5,231,668 can retrospectively prove that the relevant message was created certainly by the user, as the user assigns their signature by using their own signature key.